The present invention relates to making organic light emitting devices (OLED) which transfers organic material from a donor to a substrate.
In color or full-color organic electroluminescent (EL) displays having an array of colored pixels such as red, green, and blue color pixels (commonly referred to as RGB pixels), precision patterning of the color-producing organic EL media is required to produce the RGB pixels. The basic EL device has in common an anode, a cathode, and an organic EL medium sandwiched between the anode and the cathode. The organic EL medium may consist of one or more layers of organic thin films, where one of the layers is primarily responsible for light generation or electroluminescence. This particular layer is generally referred to as the emissive layer of the organic EL medium. Other organic layers present in the organic EL medium may provide electronic transport functions primarily and are referred to as either the hole transport layer (for hole transport) or electron transport layer (for electron transport). In forming the RGB pixels in a full-color organic EL display panel, it is necessary to devise a method to precisely pattern the emissive layer of the organic EL medium or the entire organic EL medium.
Typically, electroluminescent pixels are formed on the display by shadow masking techniques, such as shown in U.S. Pat. No. 5,742,129. Although this has been effective, it has several drawbacks. It has been difficult to achieve high resolution of pixel sizes using shadow masking. Moreover, it is challenging to align the substrate and the shadow mask, such that pixels are formed in the appropriate locations. When it is desirable to increase the substrate size, it is increasingly difficult to manipulate the shadow mask as part of the alignment process to form appropriately positioned pixels. A further disadvantage of the shadow mask method is that the mask holes can become plugged with time. Plugged holes on the mask lead to the undesirable result of nonfunctioning pixels on the EL display.
There are further problems with the shadow mask method, which become especially apparent when making EL devices with dimensions of more than a few inches on a side. It is extremely difficult to manufacture larger shadow masks with the required precision for accurately forming EL devices.
A method for patterning high-resolution organic EL displays has been disclosed in U.S. Pat. No. 5,851,709 by Grande et al. This method is comprised of the following sequences of steps: 1) providing a substrate having opposing first and second surfaces; 2) forming a light-transmissive, heat-insulating layer over the first surface of the substrate; 3) forming a light-absorbing layer over the heat-insulating layer; 4) providing the substrate with an array of openings extending from the second surface to the heat-insulating layer; 5) providing a transferable, color-forming, organic donor layer formed on the light-absorbing layer; 6) precision aligning the donor substrate with the display substrate in an oriented relationship between the openings in the substrate and the corresponding color pixels on the device; and 7) employing a source of radiation for producing sufficient heat at the light-absorbing layer over the openings to cause the transfer of the organic layer on the donor substrate to the display substrate. A problem with the Grande et al. approach is that patterning of an array of openings on the donor substrate is required. This creates many of the same problems as the shadow mask method, including the requirement for precision mechanical alignment between the donor substrate and the display substrate. A further problem is that the donor pattern is fixed and cannot be changed readily.
Using an unpatterned donor sheet and a precision light source, such as a laser, can remove some of the difficulties seen with a patterned donor. A series of patents by Wolk et al. (U.S. Pat. Nos. 6,114,088; 6,140,009; 6,214,520; and 6,221,553) teach a method that can transfer the luminescent layer of an EL device from a donor sheet to a substrate by heating selected portions of the donor with laser light. Wolk et al. comments that the use of light may be the preferred thermal transfer modality, in that it enables the precision registration needed in the manufacture of large scale devices. While laser thermal transfer does enable precision registration, it is essential that the beam of light be aligned and directed such that the correct regions of the substrate receive transferred donor material. Other means of thermal transfer would also depend upon means of precision alignment between the substrate and the thermal transfer means.
It is an object of the present invention to provide a more effective way of aligning a substrate with a laser beam, which is independent of thermal expansion of the substrate.
It is another object of the present invention to minimize the number of alignment steps needed to properly align the laser and the substrate.
These objects are achieved by a method of aligning a substrate for use in manufacture of OLED displays with a laser which produces a beam that causes the transfer of organic material from a donor element to the substrate, comprising the steps of:
(a) providing at least one fiducial mark on the substrate;
(b) positioning the substrate relative to the laser and providing relative movement between the substrate and the laser and the laser beam until the laser beam impinges upon the fiducial mark; and
(c) detecting when the laser beam impinges upon the fiducial mark and determining the position and orientation of the substrate.
It is an advantage of the present invention that, by providing fiducial mark(s) on a substrate, an effective way of determining the position and orientation of the substrate can be accomplished prior to transfer of the organic materials. It is a further advantage of the present invention that the same laser beam which is used for transfer from a donor element to the substrate can also be used to determine the position and orientation of the substrate relative to the laser. Alignment in accordance with the present invention is highly accurate and is provided by a direct detection of the fiducial marks. A further advantage of the present invention is that highly accurate alignment information can be developed with a minimum number of steps. Another advantage of the present invention is that it permits the automatic alignment of the laser beam to transfer material to different pixel sites.